Inductive sensors, e.g. variable reluctance speed sensors, use changes in magnetic flux to detect the rate at which a target moves past the sensor. The inductive sensor generally includes a permanent magnet, a pole piece or core and a coil of wire with two connections. When a tooth of a ferromagnetic gear, for example, turns past the sensor, an electromagnetic force (EMF) is induced in the coil and an electric current is driven when the EMF is connected to a load. The voltage from the EMF is induced and directly proportional to the rate of change of the magnetic flux. In the example of a rotating gear, the rotational speed is reflected on a periodic interval between the zero points of the electrical voltage and/or electrical current.
Inductive sensors may be used for incremental measurement of rotational speed, linear speed, vibrations of a structure, angular position, proximity of a target to the sensor, or any other similar characteristic of the target that passes structures periodically near the sensor. For example, inductive sensors may be used to detect the rotational speed of a crankshaft or flywheel of an internal combustion engine. The speed of the crankshaft sensed by the sensor may be used to control operation of the engine.